One major objects of the long term evolution (“LTE”) and system architecture evolution (“SAE”), which are standardized by the 3rd generation partnership project (“3GPP”), is to make a terminal compatible with a plurality of radio access technologies (“RATs”) (see FIG. 1).
Examples of a plurality of RATs include the UTRAN, GERAN, E-UTRAN, WLAN 3GPP access system and non-3GPP access system and so on, as shown in FIG. 1. These RATs all have different characteristics (e.g. coverage, transmission rate, etc.), so studies are being underway to make effective use of their individual characteristics.
FIG. 2 illustrates, conceptually, the coverage of a plurality of RATs. Furthermore, with regard to the transmission rate, the RATs may be ranked in descending order of transmission rates from WLAN, E-UTRAN, UTRAN and so on. In this way, the UTRAN of the GPRS Network, which has a wide coverage may be applied to a fast moving terminal, while the WLAN may be applied to a terminal that does not frequently moving but requires fast communication.
However, the operation of a terminal compatible with a plurality of RATs upon changing the RAT to which the terminal is connected has not been discussed enough. To be more specific, it is only between GERAN and UTRAN in a GPRS Network that the current 3GPP specifies the operation of changing the RATs.
Features upon changing the RAT in the GPRS Network include: (1) the GPRS core having the functions of the NAS (Non Access Stratum) that performs the functions of registration, call control and authentication, does not change; (2) both the UTRAN and the GERAN are systems premised upon cellular systems and one of them is selected and used; and (3) the UTRAN is developed upon the basis of the GERAN and there is a high level of affinity between the UTRAN and the GERAN.
Therefore, based on the first feature, the process of call control is not influenced when a terminal changes the RAT it uses. Furthermore, from the second and third features, change between the UTRAN and the GERAN is made by a method of selecting one of the two systems and the system is a simple one that selects one of the two systems and then disconnects the other.    Non-Patent Document 1: 3GPP TR25.331, “radio resources Control (RRC) Protocol Specification”    Non-Patent Document 2: 3GPP TR24.008, “Mobile radio interface Layer 3 specification; Core network protocols; Stage 3”